Its hard to say about prices of doing any kind of conversion nowadays. You got too many variables when doing a conversion. Such as doing the rear tail light conversion. You got..
1. Price of obtaining parts. (I.E. Tail lights, new wiring, MAYBE new rear bumper and trunk.)
2. Price of just obtaining tools (assuming you try to do some of the work) sanding material, shaving tools, cutting tools..etc.
3. Price of actual labor in itself. How much do you plan to do on your own time? Do you want a shop to cut the bumper and trunk to fit the new tail lights and then you go home and sand it up and smooth it out? OR would you rather have them do it all for you.
4. Filling gaps or making corrections after the cuts been made so that a proper fit is insured. In case you accidently cut too much or sanded off way more then you were suppose to.
5. Painting the rear bumper/trunk to match the car.
You add up
estimate prices and Its going to be at
least $1000. (1)Parts=Roughly $200-$700 depending on what you'll have to change. (2) Assuming you don't have every tool needed you'll have to buy some tools in order to work on your car. This will vary due to prices of certain tools you do not have = $5-$100 (3) Labor I have no idea on how much a body shop in your area will cost you but when I got into an accident and had my insurance paid to get it fixed they charged I believe about $80/hour for labor. (Glad my insurance covered me!

Also I got it done at a very respectable place in town.) Labor will vary soo much as some shops may take time and try to get it as correct as they can. This is where MOST of your cost will fall into. (4) This is where you might buy bondo/metal filling to fill in gaps these are usually $10-$20 a smaller bucket. (5) Painting you'll have to find a shop that'll repaint your trunk and rear bumper and that could about cost you $300 for good paint job.
So
MY ESTIMATE comes to about...$515-$1120 WITHOUT labor yet and labor I'll assume for just example lets say $1000 bucks work thats easy well over $2000 dollars for a rear conversion...
I know that isn't exactly what you're looking for, but that gives you some rough estimate. BTW whats wrong with a Gen 3 rear? I love how the gen 3 teg's rear looks they look good especially if you have the 98+ tails on your car.
Good luck with it all,
Jason